The present invention relates to seismic exploration.
Prior art seismic exploration techniques have in the past relied predominantly on the common depth point (CDP) stack technique, such as in U.S. Pat. Nos. 2,732,906 and 3,040,833.
Field arrangement of seismic detectors for the CDP were based on a compromise between preservation of vertically travelling seismic energy, containing information regarding the subsurface formations, and cancellation of horizontally travelling energy in the form of noise. Criteria used to establish detector group length in these field arrangements were: first, the length of the detector group could not be so large that reflections from dipping horizons caused time delays across the detector group with resulting signal distortion; second, the length of the detector group could not be so large that normal moveout across the detector group resulted in signal distortion. However, these two considerations limiting the length of detector groups to preserve the signal were compromised by the requirement that for cancellation of horizontal noise the detector group length had to be significantly larger than desirable for signal preservation.
Unequal weighting or spacing of sources or detectors was sometimes used in an attempt to attenuate horizontal noise. However, this technique made field techniques more complicated. Also, if the horizontal energy levels changed, such as due to a change in nature, thickness or velocity of the weathering layer, a new weighting or spacing arrangement was often required.
Another horizontal noise removal method in the CDP field technique was to specify the source-to-near detector offset to be sufficiently large for horizontal noise waves to pass at such a time to leave the geological zone of interest undisturbed by the horizontal noise. However, this method resulted in serious distortion of reflections and loss of shallow reflections where the offset was large.